libutil/experimental-features: Add pure-storepath-builtin experimenta…

…l feature

src/libutil/experimental-features.cc

@@ -62,7 +62,7 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
             Each time this derivation is built, it can produce a different
             output (as the builder outputs random bytes to `$out`).  Impure
             derivations also have access to the network, and only fixed-output
-            or other impure derivations can rely on impure derivations. Finally,
+            or other impure derivations can rely on impure derivations. Finally,pure
             an impure derivation cannot also be
             [content-addressed](#xp-feature-ca-derivations).
 
@@ -246,6 +246,18 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
         )",
         .trackingUrl = "https://github.com/NixOS/nix/milestone/39",
     },
+    {
+        .tag = Xp::PureStorePathBuiltin,
+        .name = "pure-storepath-builtin",
+        .description = R"(
+            Allow the use of [`storePath`](@docroot@/language/builtins.md#builtins-storePath)
+            in pure evaluation mode. No consensus has been found as to whether `storePath`
+            should be considered pure or impure, see the following issue:
+
+            https://github.com/NixOS/nix/issues/5868
+        )",
+        .trackingUrl = "TODO",
+    },
     {
         .tag = Xp::ParseTomlTimestamps,
         .name = "parse-toml-timestamps",

src/libutil/experimental-features.hh

@@ -37,6 +37,7 @@ enum struct ExperimentalFeature
     MountedSSHStore,
     VerifiedFetches,
     PipeOperators,
+    PureStorePathBuiltin,
 };
 
 /**